Final answer:
The molecule that connects the sugars in the sugar-phosphate backbone of nucleotide chains is the phosphate group. In disaccharides like sucrose, monosaccharides are linked by a glycosidic bond through a dehydration reaction.
Step-by-step explanation:
The molecule that lies in between the sugars in the sugar-phosphate backbone of nucleotide chains is the phosphate group. This backbone structure is a key component of DNA and RNA, forming a repeating pattern where one sugar molecule (deoxyribose in DNA and ribose in RNA) is linked to a phosphate group, which in turn is linked to the next sugar, and so on. In the formation of disaccharides like sucrose, monosaccharides such as glucose and fructose are joined by a glycosidic bond. This linkage occurs through a dehydration reaction, where a water molecule is eliminated, typically involving the OH groups on the carbon atoms of the respective monosaccharides.